[SLP]Improve compile time, NFC.

Patch improves compile time. For function calls, which cannot be
vectorized, create a unique group for each such a call instead of
subgroup. It prevents them from being grouped by a subgroups and
attempts for their vectorization.

Also, looks through casts operand to try to check their
groups/subgroups.

Reduces number of vectorization attempts. No changes in the statistics
for SPEC2017/2006/llvm-test-suite.

Differential Revision: https://reviews.llvm.org/D126476